Modern motor vehicle, gear shift systems and especially in pneumatically shifted industrial vehicle transmissions, have gear shift devices that comprise pistons and cylinders. The cylinders are, in particular, used for additional auxiliary transmissions which can be provided in the form of splitter or group transmissions, or also a combination of both transmissions. Splitter transmissions, usually front-mounted on a main transmission, divide the ratio steps between two gear steps in the main transmission while group transmissions, usually rear-mounted on a main transmission, enlarge the total spreading action of the ratio of the main transmission. For this purpose, group transmissions are often designed in the form of planetary transmissions.
In the auxiliary transmissions, the gear shift is effected via the pistons disposed in the respective cylinder which, for the purpose, is connected with the gear shift element to be engaged via a rod. Gear shift elements are usually considered synchronized gear shift elements. Synchronous total shifting times as short as possible are to be sought in the gear shifting operations. For this reason and to obtain an acceptable cold shifting characteristic, the work is carried out under high system pressure with adequately dimensioned cylinders. As a consequence of this, when the piston strikes a fixed stop surface, high dynamic forces appear in the cylinder which cause loud gear shift noises which are felt to be disturbing.
The gear shift noises and the strong impact force of the piston are to be attributed to the fact that during the synchronization operation of the transmission the piston is locked directly before its end stop but later on is loaded with the full pressure of the pressure medium so that after unlocking of the synchronizer device, with the force produced by the prestress, the selector teeth slash against each other or the piston impinges upon end stops in the cylinder.
Together with loud impact noises, the stabilization and durability of gear shift elements deteriorate under the forceful impact of the piston which can result, for example, in breakages on the gear shift fork or on the cylinder cover, to loosening of the gear shift fork connection or to failure of electronic parts.
Different means are known from the practice to attenuate the end impact of the piston in the cylinder.
The Applicant's DE 196 04 516 A1 describes a pneumatic gear shift device for a range change group gear shift which, with all its contents must also be the object of this application. The pneumatic gear shift device is furnished with a piston which in a cylinder separates from each other two working chambers. The cylinder is connected via compressed-air lines with a reversing valve for control of a compressed-air current. Between the reversing valve and the cylinder, a control valve is intercalated with a control piston and, depending on the differential pressure of the working chambers, the compressed air lines in the cylinder can be locked by the control valve. At the same time at the end of a synchronization operation, the compressed air of the working chambers of the cylinder pressurized by the piston is shut off and a residual air cushion remains therein to attenuate the end impact of the piston on a front or stop side in the cylinder. This kind of counter ventilation has not proved sufficient. Together with the expensive arrangement, the reaction time of the counter ventilation is too long.
The Applicant's DE 195 43 646 A1 has disclosed a gear shift device having one pneumatic auxiliary force for a group gear shift in which in one cylinder one piston is, likewise provided which separates two working chambers from each other. In the piston is located one shift valve which, depending on the pressurization, can be moved longitudinally relative to the longitudinal axle of the cylinder and which, upon a minimal defined volume of one of the working chambers between the piston and corresponding stop surface, impinges upon the stop surface. At the same time, the slide valve moves in a direction opposite to the direction of motion of the piston and allows a through flow of pressure medium through the piston from the pressurized working chamber to the working chamber lying in the direction of motion of the piston. This gear shift device is not sufficiently capable to break down the prestress built up during the synchronization in order to obtain an effective attenuation of the impact of the piston.
The problem in which the invention is based is to propose a gear shift device in which the impact attenuation of the shift piston is improved.
The problem is solved with a gear shift device having the characteristic features of claim 1. Developments are objective of sub-claims.